HIVE

Regarding VMs, maybe you will want to give VMware Workstation Player a try?

DDrawCompat can help with 2D and early 3D games (because Direct3D requires and relies on DDraw). I myself use it.
https://github.com/narzoul/DDrawCompat

Maybe you will want to experiment with the /3GB switch someday, which allows the user mode address space to grow to 3GB. Do note that your application has to be large address aware to benefit from this.

Uh, I do't need to try any of those things, I have already built the XP computer.

The 3GB switch trick is for your XP computer. As an example of a success story by using it:
http://neonfloppy.sytes.net/articles/2019-projectcars/

Proof that the true limits of any computer are never where one supposes they are at first glance
:)

I'm not expecting to play anything that is limited by RAM to be honest. At the moment I'm mostly having trouble getting my monitor/GPU to accept potato resolutions without stretching them to widescreen...

Monitor Assets Manager can display some internals from your monitor, directly from the hardware itself or from the registry
https://www.entechtaiwan.com/util/moninfo.shtm
Maybe that is a first step to understand what is happening with your monitor?

Unfortunately, I have no experience with DVI / HDMI / DisplayPort cables nor with any such converters. Only with good old VGA <-> VGA cables

Ooooh, that does look interesting. The nVidia control panel calls it an HDTV rather than a monitor and I reckon this is the root cause -- this tool says it can generate override INF files which might be just what I need. That'll be tonight's entertainment sorted then... ;D

Yeah, Powerstrip also used to allow to create such overrides. But I would highly suggest that you read *a lot* on that subject before attempting such a imho risky thing. Maybe using a third party software would be less risky?

Monitor Asset Manager should have provided you the name of the manufacturer of the monitor, as well as the model (cf its Plug and Play ID). It might help.

I am puzzled... Could it have anything to do with DPI scaling? Could fully analog display (VGA-VGA) make things work differently?
I wish I could help you to hopefully troubleshoot your monitor issues.

So I made MonInfo give me an INF driver which seemed legit -- all the plug 'n' play info matched what I have in the manual. Most notably, it says it does support 640x480, which the graphics card says it does not and reverts to super-stretch mode when it tries to use it. However, when I installed that INF it had no effect.

I am using HDMI and there's rumours that could be related -- my main PC is using the DisplayPort and has none of these troubles (but is a newer graphics card and on Win7 with latest drivers). I would have liked to try DVI or VGA directly but unfortunately the monitor only has DP and HDMI -- one of my fallback options is to get a new monitor that does have a DVI port, but I don't want to do that until I'm sure it'll help. I have a sinking feeling that it's the max resolution of 1920x1080 that's making the nVidia drivers decide it's an HDTV rather than a flat panel. I've tried DVI-HDMI and VGA-HDMI connectors and they haven't helped, which is why I'm suspicious.

The specs of the GeForce GT 610 say that the max resolution is not the same, depending on what one uses (Digital: 2560x1600; VGA: 2048x1536), that is why I had suggested a full VGA connection.

I would suggest that you create a thread about your issue in the Marvin/Video sub-forum of Vogons.org
https://www.vogons.org/viewforum.php?f=63
... because your issue is a technical one related to video hardware on an unsupported OS. Retrogamers are most welcome there.
:)

Maybe that one is interesting:
https://www.vogons.org/viewtopic.php?f=63&t=75570

Ooh, that one is interesting. There OP gives a link to something that didn't work for them, but sounds a lot like what I'm dealing with... another thing to try tonight! xD

Any news?
I do not have that much time to dig deeper myself, but I am definitively curious about that issue.